For better perennials, divide and conquer

This is one of my favorite times of the year — crabapples and tulips in bloom, peonies budding and perennial plants popping up
all over the garden. It is also the time of year when we realize that our
perennial gardens may have an unkempt, overgrown look to them. Plants have
multiplied beyond their boundaries and it is time to divide and conquer.

Dividing is the simple act of separating one plant into several plants. Some
signs that a plant needs divided are when the center of the clump dies out, or
flowers are smaller than normal, or few in number. Also, if the plant loses
vigor and starts to flop, or the foliage at the bottom of the plant is sparse,
it is time to divide.

Among the reasons to divide plants are to control their spread, to rejuvenate an
old plant, or to increase air circulation between plants. My favorite reason is
to increase the number of plants. Dividing can renew the vigor of a plant,
increase flower production and decrease disease problems.

A general rule is to divide perennials in the season opposite their blooming.
The best time to divide late-summer and fall-blooming perennials is in the
spring. At this time of year foliage is low to the ground and you can minimize
stem breakage. Spring and early summer blooming plants should be divided in the
late summer. During this time of year these plants are putting energy into
flower production and little energy goes to the roots.

How often a plant needs to be divided depends on the species, vigor of the plant
and how well it likes its site. If a plant is growing and blooming well and is
still within its space, it probably doesn’t need to be divided. Fast-growing perennials such as daylilies, hosta,
black-eyed susan, bee balm and mums may need to be divided every three to five
years. Some plants, such as bleeding hearts and peonies, may never need to be
divided. A few plants — including false indigo, baby’s breath, sea holly, butterfly weed and gas plant — prefer to be left alone.

Before digging existing plants, prepare the area where the new plants will be
planted. It is ideal to divide on a cloudy day when rain is in the forecast.
Prior to dividing, be sure to water the plants thoroughly a day or two before
digging. Division can be stressful on a plant and its root system. Water will
keep the roots hydrated, insuring less stress on the roots.

Use a sharp pointed spade and dig down deep, all the way around the plant. Dig
about four to six inches from the plant. Pry with your digging tool and lift
the whole clump. Be careful to leave most of the roots intact. It is not
necessary to always dig the whole clump. Some plants such as bee balm, mint and
anything that spreads by runners may be divided by digging up sections from the
outside of the planting.

Separate the clump by using your hands, a garden spade, spading fork or garden
knife. Some plants such as catmint will easily fall apart into new sections,
while others may require some muscle and a handsaw. Ornamental grasses may need
to be divided with a handsaw or sharp ax. Hostas are best divided with a sharp
spade or garden knife.

Divide the plant into clumps with three to five shoots each or about a four-inch
clump. Shoots on the outer part of the clump will have the most vigor. Discard
woody, hard and unproductive centers of plants.

Immediately replant the new clumps into the garden or into containers. Place
each division into a new hole being sure to spread out the roots. The divided
sections should be planted to their original depth. Planting too deep may
prevent flowering. Thoroughly water the new plants and don’t forget to label them.

A great book on perennial maintenance is The Well-Tended Perennial Garden by Tracy Disabasto-Aust. Dividing is an inexpensive way to gain additional
plants for your garden and can also be shared with friends.

Jennifer Fishburn is horticulture educator with the Sangamon-Menard Unit of the
University of Illinois Extension. Contact her at fishburn@illinois.edu.